"Selective publication can lead doctors to make inappropriate prescribing decisions that may not be in the best interest of their patients and, thus, the public health," say Turner and team.

The idea that unfavourable test results get quietly tucked away so nobody will see them - sometimes call the "file drawer effect" - has been around for years.

Turner and colleagues were able to study the question because the US Food and Drug Administration has a registry in which companies are supposed to log details of their drug tests before the experiments are begun.

They could see which experiments approved by the FDA between 1987 and 2004 were ultimately publicised in the medical literature and the main criteria the researchers planned to measure success.

"It tells you where they placed their bets before they saw the data," Turner says.

Of the 74 studies that started for the 12 antidepressants only 38 produced positive results for the drug. All but one of those studies were published.

However, when it came to the 36 studies with negative or questionable results, as assessed by the FDA, only three were published and another 11 were turned around and written as if the drug had worked.

"Not only were positive results more likely to be published, but studies that were not positive, in our opinion, were often published in a way that conveyed a positive outcome," say Turner and colleagues.

For example, of the seven negative studies done on GlaxoSmithKline's Paxil (sold in Australia as Aropax), five were never published.

There were five studies for Pfizer's Zoloft, but the three showing the drug to be ineffective were not published and a fourth study, ruled as questionable by the FDA, was written and published to make it appear that the drug worked.

Drug companies respond

A Glaxo spokeswoman said the company posts the data from all of its trials, positive or negative, on the Internet.

"GlaxoSmithKline agrees that public disclosure of clinical trial results for marketed medicines is essential and fully supports registration of all trials in progress," she says.

Pfizer was not immediately available to comment.

Turner and his colleagues did not find out who was to blame for not publishing the studies.

Although the authors and drug company sponsors may not have submitted the unfavourable results for publication, medical journals and their editors may have played a role by deciding they would rather publish favourable results.

"There's an expectation that if you get a positive result, that's what you're supposed to do, and if you get a negative result you have failed," says Turner.

"The first impulse is to say, 'I was wrong. Maybe I should move on to something more interesting'" so the results may never get written up.